I've had the game for about a year or more now, and my original characters from when I bought the game (well before Hearth and Home), are still playable. I expect at the very least our saves will be useable until a full official release (and even then likely possible).

As for new biomes, I'm not sure how that will work. But currently there are 3 biomes already in the game (so they are spawned in the map), but they are essentially empty. They have their decorations and such, but no active monsters or anything. I imagine any content updates that deal with these biomes will just add more events/ai and such to the already spawned biomes.

For example, you can end up finding the Mistlands biome, but there's nothing spawned there. Just the landscape and doodads. It looks really cool, btw! Can't wait till it's actually populated with monsters.